Hydraulic equipment, and in particular high performance pumps and motors used in applications where there is a large amount of particulate contaminant material are found to be subject to an unusually high degree of wear. The problem is particularly troublesome in underground mining operations where maintenance is difficult to accomplish and equipment is frequently run on a round-the-clock basis. Not infrequently, hydraulic pumps will last no longer than about three months due to wear caused by the presence of highly abrasive particulate contaminants in the hydraulic fluid. The particulate contaminants in the hydraulic fluid abrade and erode critical sealing surfaces within the pumps and motors and at the end of a relatively short time, volumetric efficiency has dropped to the point where the pump or motor is very inefficient. This operating inefficiency, among other things, leads to overheating of the hydraulic fluid and eventually to failure of the pump. Because the occurance of such failures is relatively unpredictable and inspection and monitoring of pump efficiency is not easily carried out, particularly in underground mines, the practice has developed to simply replace the hydraulic pumps and motors at relatively short intervals in order to avoid problems.
Significant reductions in wear due to contaminated hydraulic fluid are achieved through the use of gear pumps and motors constructed in accordance with the teachings of copending application Ser. No. 29,891. In that application a construction is disclosed in which radially movable sealing elements provide a sealing surface with the tips of the gear teeth adjacent the high pressure port of the pump or motor. These elements are pressure compensated to eliminate slippage caused by wear at the tips of the teeth by maintaining a good seal with the tips of the teeth despite wear which may occur due to the presence of contaminants in the hydraulic fluid.
Another leakage path in gear pumps and motors is between the sides of the teeth and the side walls of the pumping chamber. It is known in the prior art that the side walls should be comprised of pressure compensated plates which are pressed against the sides of the teeth by fluid pressure which is communicated against the outsides of the plates from the high pressure side of the pump or motor.
Another type of leakage path which is aggravated at high pressures exists in certain regions of the pressure plates interfacing with the gears where there is an abrupt transition from high to low pressure.
One such leakage path exists at the surface of the plates bounding the mesh region of the gears. Where the gears mesh there is a point of contact which moves along a path termed the path of contact which is the demarcation line between high and low pressures. Although there is little or no leakage between the teeth when the gears are rotating, the points along this line at the boundaries of the teeth have been discovered to be natural leakage paths in which the pressure plates are subjected to a higher degree of wear and abrasion than the rest of their surface. So far as we can determine, the prior art contains no solution to the problem other than careful maintenance and avoidance of oil contamination by use of filters or the like.
The other point where a large pressure drop may exist is between the root circle of the gears and the shaft bearings. Although the existence of this leakage path has been recognized, where journal bearings have been employed it has been considered to be necessary for lubrication of the bearings or at least an expedient solution to the lubrication problem.
Even though the side plates are pressure compensated so as to keep the side plates pressed against the sides of the teeth despite wear of the side plates, they are still susceptible to damage by abrasive and erosive action of contaminants in the hydraulic fluid in the sealing area between the root circle of the teeth and the shafts on which the gears are mounted.
The susceptibility to damage of the type to which this invention relates is attributable to the fact that the surface of the pressure plate adjacent the teeth must be selected primarily for its ability to act as a bearing for the sides of the gear teeth which normally move at very high velocities. Available materials (primarily bronzes) that make good load bearing surfaces are relatively soft and offer little resistance to abrasive and erosive attack by particulate contaminants. As indicated above, pressure compensation of the side plates is a substantial solution to the problem of wear between the sides of the teeth and the pressure plate. However, it has been discovered that pressure compensation does not provide a solution to wear of the side plate in the teeth mesh region or interiorly or beneath the root circle of the teeth and in fact these areas are the ones most susceptible to damage by contaminants in gear pumps or motors.
Various other approaches to the problem of erosion and abrasion at the root circle are currently in use. One approach is to increase the root seal, thereby reducing the amount of leakage by increasing the distance between the root circle and the outside diameter of the gear shaft. The problem with this approach is that it can only be accomplished by reducing shaft diameter which leads to bearing failure or by increasing the overall size of the gears which also means an increase in the size of the pump housing and an appreciable increase in weight and in cost. Another approach has involved the pressurization of the ends of the housing so that there is no flow of oil into the bearing region. Attendant with this approach is an increase in the cost of the housing or a risk of increased pressure failure. Shaft sealing problems are also more difficult. Pressure compensation of the side plates is more difficult to accomplish due to difficulties in isolating high and low pressure regions at the back of the side plates. The bearings required for such applications must be roller or needle bearings.